Policy Based Routing Using Route Maps

Route maps are very similar to access lists. They both perform "if then" programming, in that they state criteria that is used to determine whether specific packets are to be permitted or denied.

The main difference is that the route map has the additional capability of adding a set criterion to the match criterion. In an access list, the match criterion is implicit; in a route map, it is a keyword. This means that if a packet is matched to the criterion given in the route map, some action can be taken to change the packet.

For example, an access list could state something similar to this logic: If the cupcake is lemon-flavored, keep it, but if it is has walnuts in it, throw it away.

Along the same lines, a route map could specify logic such as this: If it is a lemon-flavored cupcake, ice it with lemon butter frosting. If it has walnuts, then ice it with melted chocolate. If it has neither a lemon flavor nor walnuts, leave it alone. The route map is obviously more powerful because it can change the entity.

Now to make this simple example slightly more complex, to show the additional complexity of route maps, add a logical AND and a logical OR. For example, if it is a lemon-flavored cupcake AND it contains poppy seeds, ice it with lemon butter frosting. If it has walnuts OR it was baked today, then ice it with melted chocolate. If it has neither a lemon flavor nor walnuts, leave it alone.

The route map would look something like this:

route map cupcakes permit 10 match lemon flavored poppy seed set add lemon butter frosting route map cupcakes permit 15 match walnuts match baked today set melted chocolate frosting route map cupcakes permit 20

For the mathematicians among you, this could be written as follows:

Set nothing

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